Vacuum brazing with a brazing material adjacent the joints to be formed and sufficient heat results in the capillary flow of the brazing material into the joints in a similar fashion to conventional atmospheric brazing.
Many base metals such as aluminum are difficult to braze but can be brazed satisfactorily using vacuum brazing processes. These difficult to braze metals and their alloys inhibit the wetting and flowing of brazing alloy under ordinary conditions and usually require special procedures employing fluxes in order to accomplish satisfactory joints.
Aluminum and other difficult to braze metals form surface compounds both prior to and as the metals reach the brazing temperature which detract from and are incompatible with the bonding or brazing alloy. These surface compounds include oxides of the metals which particularly inhibit wetting by the brazing material. Therefore, the prevention of this oxidation or the removal of the formed oxides is necessary for the production of satisfactory joints in these difficult to braze metals and for many years fluxing of the surface of the metals was thought necessary to remove these formed oxides.
It has been found, however, that the use of the flux can be eliminated in some cases in the brazing of difficult to braze metals, such as by the use of a reducing atmosphere such as hydrogen. However, this brazing process has not been very successful due to the difficulty and expense in obtaining the free flow of the reducing gas throughout the structure which is to be brazed.
In vacuum brazing, most of the surface oxides are cracked and displaced by molten filler metal under the high temperature and extremely low pressure conditions employed. Thus, some of the oxides are disassociated from the metals under normal vacuum brazing conditions by vaporization.
It has been found that in the vacuum brazing of aluminum, for example, and particularly in the vacuum brazing of aluminum assemblies with an aluminum-silicon braze alloy, that the addition of magnesium to the brazing alloy aids gettering the furnace envelope and possibly promotes alloy wetting and brazing. While the complete role of magnesium is not completely understood, it is known that the presence of magnesium vapor acts to beneficiate or "getter" the vacuum by reaction with the oxygen and water vapor present at all times, to some extent, in all practical vacuum brazing furnaces.
In the usual case, a small quantity of magnesium, i.e. about one percent, is added to the aluminum-silicon braze alloy so that prior to and during the melting and flow of the braze alloy, magnesium vapor is released to getter the atmosphere in the region of the brazement and in fact the whole of the vacuum chamber, depending upon the chamber size and amount of magnesium vaporized. The vaporization of the magnesium appears not only to assist in eliminating the requirement for any flux, but also possibly permits the use of a somewhat higher pressure during the brazing operation.
To achieve these beneficiating effects from the magnesium vapor, the quantity and timing of the vaporization thereof must be controlled. In the vacuum brazing of smaller assemblies, such as air conditioning evaporators, there is generally a considerable excess of magnesium released and the resulting vacuum beneficiation adequately promotes alloy flow and wetting to easily form fillets in the joints of adequate size and strength. In these small assemblies, the entire unit approaches brazing temperature at the same time and hence the release of magnesium vapor occurs throughout the assembly at all joints at about the same time.
However, the control of magnesium vaporization in very large brazed aluminum industrial heat exchangers has been found to be quite difficult. Because these assemblies consist of large relatively dense arrays of plates, fins and side bars having very few external openings, the rate of heating by radiation through the exchanger is quite slow so that the exterior surfaces of the assembly approach brazing temperature well ahead of the internal joints in the assembly. The tightly stacked side bars and plates on the exterior of one of these large assemblies present only a small amount of brazing alloy directly to the furnace interior since the brazing alloy is commonly formed in brazing alloy sheets pre-assembled directly in the exchangers. Thus, during brazing of these large assemblies the exterior joints and their fillets are formed significantly before the interior-most joints in the assembly. The result of this differentially timed brazing is that during the early critical portion of the brazing cycle when the exterior joints are being brazed, there is a lack of beneficiating magnesium vaporized because only the surface joint magnesium is vaporized at this time, and this results in a poor quality of brazing of the exterior joints. This poor quality is believed to result from this rather small quantity of magnesium attempting to "getter" the entire relatively large furnace volume so that no magnesium or relatively little is available to beneficiate the area of the joints and reduce the formations of oxides at the joints and promote alloy wetting when the brazing temperature has been reached.
It is a primary object of the present invention to ameliorate the above noted problems in the vacuum brazing of certain base metals in large assemblies in the presence of a brazing alloy containing a gettering agent.